Conventionally, an image forming apparatus, such as a copying machine, a printer, a facsimile, or a multifunction peripheral, is provided with a plurality of rollers for conveying a sheet member on which an image is to be formed. These rollers are driven by a drive motor. As the drive motor, a servomotor such as a DC brushless motor may be used. There is known a configuration in which a DC brushless motor is used as a drive motor for driving a conveying roller that conveys sheets from a sheet feed cassette (see PTL 1).
Usually, an image forming apparatus that uses the servomotor as a drive source for the conveying roller is provided with a detector such as a rotary encoder for detecting the rotation speed and the like of the drive motor. In addition, a motor driver is electrically connected to the servomotor, and a controller such as a CPU is electrically connected to the motor driver. Then, the controller outputs, to the motor driver, a control signal representing a commanded speed to the servomotor. The motor driver generates a drive current by using a PWM (Pulse Width Modulation) method, on the basis of the commanded speed represented by the control signal and an actual rotation speed represented by a detection result from the detector, and supplies the drive current to the servomotor.
Meanwhile, as the control signal outputted from the controller to the motor driver, a pulse signal including a plurality of pulses each having a pulse width corresponding to the commanded speed to the servomotor is used. In this case, the motor driver detects the cycle of the pulse signal and the commanded speed, on the basis of a rising edge or a falling edge (hereinafter these edges are referred to as pulse edges) of the pulse signal.